In many communication applications there is a need for establishing a secure communication link between two communication devices. A common example of such a need exists between cellular telephones and wireless headsets which communicate with each other via the Bluetooth® protocol. Establishing a secure communication link between two communication devices protects against passive eavesdropping by unauthorized parties and against active eavesdropping (known as man-in-the-middle attacks).
A number of different methodologies are known for establishing secure communication links between two wireless communication devices. One method commonly used in conjunction with the Bluetooth® communication protocol, known as Simple Pairing, uses strong encryption algorithms to provide users with protection against passive and active eavesdropping. Using strong encryption algorithms complicates the task of a would-be eavesdropper, since such eavesdropping requires solving a difficult public key problem in order to derive the link key for a communication session.
User interaction with the communication devices may be necessary to protect against the man-in-the-middle attack. A man-in-the-middle attack occurs when a user wants to connect two devices, but instead of connecting directly with each other, the devices connect to a third (attacking) device that plays the role of the other device with which each device is attempting to pair. The attacking device then relays information between the two communication devices giving the illusion that they are directly connected. The attacking device may eavesdrop on communications between the two devices (known as active eavesdropping) as well as insert new or modify information and commands communicated over the link.
Current methods for secure pairing are outlined in the Simple Pairing Whitepaper published by the Bluetooth® Special Interest Group (Rev. V10r00, pub. 2006-08-03) which describes the Simple Pairing feature in the Lisbon release of the Bluetooth Core Specification, the entire contents of which are hereby incorporated by reference. Two current methods seek to defeat a man-in-the-middle attack require the user seeking to establish the secure pairing between two communication devices to take an action. Specifically, the current methods require the user to confirm that a six-digit number displayed on one device is the same as the six digit number displayed on the other device (known as “Numeric Comparison”), or type a six digit number displayed on one device into the second device (known as “Passkey Entry”). User confirmation or entry of the six digit number enables the user to verify that the two communication devices are talking directly to each other and not via a third device. The six digit number is an artifact of a security algorithm that provides no benefit in decrypting subsequent encoded data exchanges. (A third method uses another “out of band” communication link to exchange or transfer cryptographic numbers used in the pairing process, but this method requires the devices to include two different types of transceivers.)
While the Numeric Comparison simple pairing method provides strong protection against man-in-the-middle attacks, the method requires that both communication devices include displays which can show the six digit number. Similarly, the Passkey Entry requires one device to include a display that can show the six digit number and the other device to include a numeric keypad to receive the passkey entry. Thus, these methods may be unsuitable for simple or small communication devices that do not have room or the processing power to support such a display or keypad.